In general, in synchronous machines, when the number of slots is to be selected, the number of slots per pole per phase is often made in an integral number (hereinafter referred to as “integral-number slots”). On the other hand, from a viewpoint of formation of an improvement waveform, and flexibility of design, the number of slots per pole per phase is made in a fractional number (hereinafter referred to as “fractional-number slots”) in some cases.
Although the fractional-number slots have an advantage that an excellent waveform can be formed, and a high degree of design freedom can be obtained, the number of slots per pole becomes unequal, and hence harmonics are generated in the armature magnetomotive force in addition to a fundamental wave. A harmonic magnetic flux caused by the magnetomotive force acts as an exciting force under certain circumstances, and becomes a cause of the vibration and noise of the armature core (stator core) in some cases.
For example, when fractional-number slots are applied to a multipolar machine (generally a machine with the number of poles of which is 8 or more) having a larger number of poles as can be seen in a hydroelectric power generation system, a harmonic called a sub-synchronous (or fractional) harmonic of a case where the number of slots per pole per phase (number of slots or coils per each pole per each phase), which is referred to as Nspp, includes a fractional number, and a denominator thereof takes a value (for example, 5 or 7) greater than or equal to 4 becomes liable to occur. A state where a frequency of the electromagnetic force based on such a harmonic and fundamental wave is close to a natural vibration frequency is called a resonance, and this becomes a cause of vibration of the armature core.
It should be noted that there is a case where coil arrangement is carried out by replacing one coil piece in a phase belt with a coil piece in an adjacent different phase for each of both upper and lower coil pieces. However, this arrangement method, which is called the interspersed one coil windings, is contrived to reduce the armature magnetomotive force of harmonics such as a fifth-order harmonic, seventh-order harmonic, and the like in the integral-number slots, and is not intended to reduce the harmonic, i.e., the sub-synchronous (or fractional) harmonic in the fractional-number slots.
Under the circumstances, it is desired to provide the armature windings of a rotating electrical machine capable of reducing a specific harmonic generated in fractional-number slots be provided.